Update prepare-functest role for k8s and fix fetching xci.env

Update prepare-functest role for k8s and fix fetching xci.env

This change updates prepare-functest role for testing k8s scenarios
using functest healthcheck. The changes include
- update tasks to skip checking/creation of public gateway which
is needed for OpenStack based scenarios
- update run-functest.sh.j2 template and set the used docker image
name based on FUNCTEST_SUITE_NAME that is going to be used
- update run-functest.sh.j2 template and add commands needed to run
tests using functest-kubernetes-${FUNCTEST_SUITE_NAME} docker image
- update env.j2 to exclude setting the var EXTERNAL_NETWORK which is needed
for OpenStack based scenarios

Apart from updating the the prepare-functest role, a bug has also been fixed
by adding the fetching of xci.env for installer kubespray.

installer-type:kubespray
deploy-scenario:k8-nosdn-nofeature

Change-Id: Ia701db9748ea9509a2dc165341285fb189aa7266
Signed-off-by: Fatih Degirmenci <fdegir@gmail.com>